The prior art, exemplified by U.S. Pat. No. 3,697,666, issued Oct. 10, 1972, entitled "Enclosure for Encapsulating Electrical Components" by Wakely et al., typically employs a combination of a ceramic substrate, a metal leadframe, and a devitrifying glass composition which forms a glass-to-metal seal. Conventionally, the components of such a package are placed in the proper orientation for assembly while the sealing glass remains substantially non-devitrified. The semiconductor components are then assembled into the package. The final enclosure seal, exemplified by U.S. Pat. No. 3,768,991, issued Oct. 30, 1973, entitled "Method for Sealing an Enclosure for an Electronic Component" by Bryant C. Rogers, and assigned to Diacon, Inc., typically employs heating the package and the cap to a temperature to allow the devitrifying glass to form a seal when the package and lid come in contact.
The disadvantages of the system of the prior art are: (1) high seal temperatures required for seal can damage current semiconductor components and are above the Gold-Silicon eutectic temperature (363.degree. C.) used for semiconductor component attachment to the package which can result in semiconductor failures, (2) seal time is too short to assure that the seal glass attains full devitrification to assure a strong package seal, a particular concern for large seal areas such as high pin count packages, (3) moisture and contamination levels in the sealed enclosure cavity are high and do not meet current high reliability requirements.
The high seal temperature and package strength problems can be resolved by changing from devitrifying glass to vitreous glass. However, the moisture and contamination problems remain and additional problems are introduced. Vitreous glass, unlike devitrifying glass, will melt and reflow each time it is exposed to its melting temperature. This means that during the semiconductor assembly process (i.e. die attach, wire bonding, seal) the enclosure leadframe can shift resulting in the breaking of the tiny metal wires connecting the semiconductor component, the enclosure metal leadframe, and thus ruining the device. This shifting problem was resolved by the invention described in U.S. Pat. No. 4,141,712, entitled "Manufacturing Process for Package for Electronic Devices", issued Feb. 27, 1979, to Bryant C. Rogers and assigned to Diacon, Inc. In that system devitrifying glass was used. Again, if devitrifying glass is used, the high seal temperature and package strength problems and the other disadvantages remain.
What is required is a new package system with low seal temperatures, low moisture and contamination levels, no lead shifting during semiconductor component assembly, and optimum post seal strength.